Aminocarboxylic acid salts can be synthesized by a number of known methods. Previously, the salts of certain aminocarboxylic acids had been converted to aminocarboxylic acids by using conventional acid crystallization methods or by electrochemical processes. These methods were expensive and/or not well suited because of the relatively high water solubility of the acid in the presence of sodium ions. Thus, using conventional methods, the production of aminocarboxylic acids cost about three times what their corresponding salts cost.
Yet, there is a strong need for these aminocarboxylic acids, including for example health and industrial uses. Further, N-(hydroxyethyl)ethylenediaminetriacetic acid (HEDTA acid) has general broad applications as an industrial cleaner, for boiler cleaning and treatment, and as a micronutrient. Further, technology is needed to recover HEDTA acid from spent boiler cleaning applications.
An alkali metal salt of aminocarboxylic acid is an aminocarboxylic acid which has had one or more of its ionizable hydrogen atoms (on a COOH group) replaced by an alkali metal such as sodium or potassium to form a salt. Thus, aminocarboxylic acids salts are converted into their respective aminocarboxylic acids, including N-(hydroxyethyl)-ethylenediaminetriacetic acid (hereinafter referred to as HEDTA acid), diethylenetriaminepentaacetic acid, hydroxyethyliminodiacetic acid, nitrilotriacetic acid, and glycine. These structures are very large when compared to monovalent alkali metal ions such as sodium and monovalent anions such as chlorine. This size differential makes diafiltration possible. Diafiltration is a constant volume washing process in which a membrane is used to selectively remove smaller molecules and ions, and/or exchange solvent while retaining the larger molecules and ions in the feed solution.
The following patents are representative of the pertinent art. U.S. Pat. No. 4,889,663 issued Dec. 26, 1989 to Pfenninger. It discloses the use of diafiltration to separate waste water into a disposable organic free aqueous permeate effluent portion and an aqueous retentate portion that is concentrated with organic compounds.
U.S. Pat. No. 4,818,409 issued Apr. 4, 1989 to Puetter et al. discloses a method for obtaining aqueous solutions of organic acids or bases which uses an ion exchange membrane to obtain aqueous solutions from salts of the same.
U.S. Pat. No. 4,765,905 issued Aug. 23, 1988 to Kitamura et al. discloses a method for recovering acids from an aqueous solution of a mixture of the acids and metal salts by dialysis using a fluorinated anion exchange membrane made of a copolymer having repeating units of a given formula.
Japanese Patent No. J5 8058-112 issued Apr. 6, 1983 to Tokuyama Soda discloses the use of a multistage dialysis to reclaim or purify various inorganic and organic acids from their metallic salts. Another Japanese patent, J6 0132-604-A, issued Jul. 15, 1985 to Toray Engineering discloses a method for concentrating and recovering valuable organic compounds also containing inorganic salt by desalting using membrane-separation treatment under an operating pressure above the osmotic pressure of the aqueous solution. Dilution water is added during the treatment to keep the settled level of the starting tank. This patent thus discloses a salt removal process.
Such processes represent older, more costly, less efficient ways of converting aminocarboxylic acids salts to aminocarboxylic acids. The present invention is directed to an improved method of converting aminocarboxylic acid salts.
It would be desirable to provide an improved method of converting aminocarboxylic acid salts to aminocarboxylic acids. It would be advantageous to have a cheap, efficient, environmentally safe, easily applied, and improved method of converting aminocarboxylic acid salts into their corresponding aminocarboxylic acids. The present invention is directed to this need.
It would also be desirable to provide an improved method of converting VERSENOL .TM. 120 (the trisodium salt of N-(hydroxyethyl)-ethylenediaminetriacetic acid) to HEDTA acid.